Goals: GRAIL flew twin spacecraft -- named Ebb and Flow -- in tandem around the moon to precisely measure and map variations in the moon's gravitational field. The goal was to reveal differences in density of the Moon's crust and mantle and will help answer fundamental questions about the Moon's internal structure, thermal evolution and history of collisions with asteroids.

Accomplishments: The twin GRAIL probes orbiting Earth's moon generated the highest resolution gravity field map of any celestial body. The gravity field map revealed an abundance of features never before seen in detail, such as tectonic structures, volcanic landforms, basin rings, crater central peaks and numerous simple, bowl-shaped craters. Data also show the moon's gravity field is unlike that of any terrestrial planet in our solar system. The map will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved.

At the end of an extended mission, Ebb and Flow were sent purposely into the lunar surface because their low orbit and low fuel levels preclude further scientific operations. They impacted at mountain near Goldschmidt crater on the lunar near side.